Visible V-8 - an old modeler's staple. Originally tooled by Renwal, Revell-Monogram has the tools now and is still in the re-issue rotation. The also have the tools for the Visible Airplane Engine (R-1340 Wasp), which hasn't been re-issued for years. A friend of mine still has an original Renwal Visible Airplane Engine in the box, which he never started to build.

(I missed a chance to buy one twenty years ago as I was out with my nephews; I saw one and thought I would buy it a week later. It was gone!) The 1/3 visible Wankel engine is worth quite a bit. Haynes/Airfix have the four cylinder,V-8 models on the market, but I think the most impressive for kids is the jet engine that they do.